In many applications of lasers or other radiation sources it is important to prevent reflected radiation from interacting with the source, since such interaction can, for instance, generate noise and unwanted feedback. An example of an application in which there frequently is need to isolate a source from reflected radiation is lightwave communications, especially high bit rate communications over relatively long distances.
It has long been known that the Faraday effect in magneto-optic materials can be used to provide a non-reciprocal device that can serve as an isolator, i.e., a device which can permit light passage in only one direction. Yttrium iron garnet (YIG) is a magneto-optic material that has been used for isolator applications. However, YIG has recently become quite expensive. Furthermore, it has only a relatively small specific rotation in the near infrared wavelength regime of interest for lightwave communications (e.g., 0.8-1.6 .mu.m), such that a large path length (about 2.7 mm at .lambda.=1.31 .mu.m) is required to provide the 45.degree. rotation necessary for an isolator. It also has a relatively high saturation magnetization, which typically requires the use of a large, high field magnet (e.g., SmCo) that typically is not only expensive but also may affect, and/or be affected by, nearby components.
It is known that Bi substitution in rare earth iron garnets can greatly increase the specific Faraday rotation of these materials. It is also known that the specific Faraday rotation of garnets of interest herein typically is a function of temperature, decreasing with increasing temperature.
A magneto-optic isolator typically comprises first and second polarizing means, a magneto-optic member between the polarizing means, and magnet means adapted for causing magnetic saturation of the relevant part of the magneto-optic member. Radiation from a radiation source (e.g., a semiconductor laser or a LED) is linearly polarized by the first polarizing means, the plane of polarization is rotated by an angle of 45.degree..+-..theta. by the magneto-optic member, and only radiation polarized parallel to the direction of polarization of the second polarizing means passes through the second polarizing means and is available to be utilized by appropriate utilization means. Typically the polarization direction of the second polarizing means is set 45.degree. from the direction of polarization of the first polarizing means, such that, for .theta.=0, essentially all radiation that has been passed by the first polarizing means is also passed by the second.
Radiation reflected back towards the isolator encounters first the second ("exit") polarizing means, which will pass only the component parallel to its direction of polarization. This radiation will be rotated by 45.degree..+-..theta. by the magneto-optic means, and thus be oriented 90.degree..+-..theta. to the polarization direction of the first ("entrance") polarizing means, which will pass only the electric field component proportional to sin .theta.. The back reflected radiation intensity passed by such an isolator will therefore be proportional to sin .sup.2 .theta.. For .theta.=0, the isolator blocks all reflected radiation. However, the specific rotation of the garnet materials of interest herein being temperature dependent, .theta. typically is zero only at one particular temperature within the operating temperature range of the isolator.
The ability of an isolator to block reflected radiation is frequently expressed in terms of the "extinction ratio" (ER), where ER (in db)=-10 log (P.sub.2 /P.sub.o). P.sub.o is the incoming intensity parallel to the second polarizing means and P.sub.2 is the outgoing intensity from the first polarizing means towards the radiation source. Assuming ideal polarizing means, ER=-10 log (sin .sup.2 .theta.), which is shown in FIG. 1. Clearly, ER is a sensitive function of .theta., which in turn is a function of the temperature.
In many important applications of optical isolators ER may not be less than a specified value over a relatively wide temperature range (e.g., 0.degree.-85.degree. C. in exemplary optical fiber communication apparatus). Thus, the temperature dependence of the specific rotation of magneto-optically active garnet materials is a significant drawback.
Workers in this field have proposed several approaches to reducing this drawback. However, the prior art approaches have not proven fully satisfactory. For instance, it has been proposed to use two serially connected conventional isolators that have slightly different characteristics (S. Takeda et al., Conference on Lasers and Electro-Optics, Anaheim, Calif., Apr. 25-29, 1988, paper W4-02). This approach is costly and results in a relatively complex and large isolator package. It has also been proposed to use a composite structure comprising two types of Bi-substituted rare earth iron garnet thick films which have opposite Faraday rotations, such that the resulting rotation, the sum of the two contributions, is relatively temperature independent. (See K. Machida et al., Optoelectronics-Devices and Technologies, Vol. 3(1), pp. 99-105; H. Minemoto et al., Proceedings of the International Symposium on Magneto-Optics; Journal of the Magnetics Society of Japan, Vol. 11, Suppl. S1, pp. 357-360; K. Matsuda et al, Applied Optics, Vol. 27(7), pp. 1329-1333). This approach requires the use of two relatively thick garnet films of precisely controlled composition and thickness. Production of these films is difficult. Furthermore, such an isolator can be expected to have relatively high insertion loss, due to the relatively long pathlength in garnet films.
In view of the technological importance of magneto-optic isolators, it would be highly desirable to have available a technique for temperature compensating such an isolator that is not subject to the shortcomings of prior art techniques. In particular, it would be desirable to have available magneto-optic members that can be produced relatively simply and inexpensively, that can result in isolators having relatively short pathlength in the garnet material, and that have a relatively large ER over a relatively large temperature range. This application discloses apparatus comprising such members.